As a quick update, here're some articles that describe what the "Distribute Upgrades over X days" option does:
http://www.symantec.com/docs/TECH97073
http://www.symantec.com/docs/TECH179006
Oh, I think the below bits got lost in the mulitple post updates. Not sure if you've seen these before, but very useful if upgrading clients in remote sites:
Don't forget you can use this uprade function with the ability to host an installation package on any ol' webserver.
This means if you have an IIS box on any remote site, you can point the clients in that site to their local IIS box for the upgrade package instead of the SEPM (see below articles)
http://www.symantec.com/docs/TECH96873
http://www.symantec.com/docs/TECH97406
There's a win2k8 one somewhere too but can't find it for the life of me